% start out with plain.tex,
% having TFM-based CM fonts preloaded in \fam 0...3

% load OT math font in \fam 4
\input luaotfload.sty
\font\4={name:XITS Math:mode=base;script=math} at 10pt
\font\5={name:XITS Math:mode=base;script=math;+ssty=0} at 7pt
\font\6={name:XITS Math:mode=node;script=math;+ssty=1} at 5pt
\textfont4=\4 \scriptfont4=\5 \scriptscriptfont4=\6

% set up macros to use math font in \fam 4, just enough for this test
\def\x{\Umathchar"7 "4 "1D465 }
\def\y{\Umathchar"7 "4 "1D465 }
\def\3{\Umathchar"7 "4 "0033 }

%\tracingall
$$\Uradical "4 "221A {\x}$$
$$\Uroot    "4 "221A {\3}{\x^\3+\y^\3}$$
\bye